What proof do you have that CX changed your flight?

Your HKG-DFW was on AA and potentially AA’s computer moved you to the next day’s HKG-DFW as AA worked out you would miss the original due to the delay ex SIN.

Your HKG-LAX flight was on AA so you would likely have been under AA airport control.

I have this feeling that the delay by CX ex SIN has coloured your view.

They offered you 2 different re routes and gave you your preferred one even though it took longer and the first one they offered was shorter.

You complain about delays now which would have been mitigated by taking the UA flight.

Sure middle seat is not ideal but AA could have put you into a middle seat as well - remembering that seat allocation is not guaranteed.